Device for the nondispersive infrared gas analysis

ABSTRACT

This invention comprises a device for detecting and measuring a gas component of a gas mixture using the optical-pneumatic effect produced by absorption of infrared radiation by a gas. The device comprises, in optical alignment, a radiation source, filter means, a cuvette system and a receiver unit which contains a measuring chamber and a comparison chamber. Pressure differential means such as a diaphragm condenser measures the opticalpneumatic effect produced by the absorption of radiation. The device is provided with porous means for flow of gases to eliminate interfering fluctuations in gas flow.

United States Patent Luft [ Oct. 24, 1972 [72] Inventor:

' [s41 DEVICE FOR THE NONDISPERSIVE INFRARED GAS ANALYSIS Karl FriedrichLutt, am Wunnesberg 32, Essen, Germany [22] Filed: May 12, 1971 [211Appl. No.: 142,616

Related U.S. Application Data [63] Continuation-impart of Ser. No.745,197, July 16, 1968, abandoned.

[52] U.S. Cl. ..250/43.5 R, 356/95 [51] Int. Cl. ..G0ln 21/26 [58] Fieldof Search ..250/43.5 R; 356/95 [56] References Cited UNITED STATESPATENTS 3,227,873 1/1966 Liston ..250/ 43.5

Primary Examiner-James W. Lawrence Assistant ExaminerC. E. ChurchAttorney-Malcolm W. Fraser 7] ABSTRACT This invention comprises a devicefor detecting and measuring a gas component of a gas mixture using theoptical-pneumatic effect produced by absorption of infrared radiation bya gas. The device comprises, in optical alignment, a radiation source,filter means, a cuvette system and a receiver unit which contains ameasuring chamber and a comparison chamber. Pressure differential meanssuch as a diaphragm condenser measures the optical-pneumatic effectproduced by the absorption of radiation. The device is provided withporous means for flow of gases to eliminate interfering fluctuations ingas flow.

8 Claim 4 Drawing Figures swim, 7IIIIIIIIII YIIJ VIIIIIIIII) 7IMPATENTEflom 24 I972 3 7 O0 891 sum 1 nr 2 V/// ///l 1/ A 32 INVENTOII QKARL FRIEDRICH LUFT 28 ATTORNEY DEVICE FOR THE NONDISPERSIVE INFRAREDGAS ANALYSIS This application is a continuation-in-part application ofcopending application Ser. No. 745,197, filed July 16, 1968 nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to a device for theanalysis of gas mixtures and more particularly, to a device formeasurement of the optical-pneumatic effect produced through theabsorption of infrared radiation in a gas mixture.

The optical-pneumatic effect has been known since the experiments ofTyndall and Rontgen. In many fields of technology, nondispersive,infrared gas analysis has been employed and is successful in thedetection of an individual component in a gaseous mixture. Attempts havebeen made to broaden the measuring method in such a manner that thedetermination of several components of a gaseous mixture by the use ofone apparatus becomes possible. One such device employed rotatablymounted filters and analysis chambers to determine three components in agaseous mixture [K.F. Luft, Angew. Chem, Applied Chemistry, Edition B,p. 19, 2/12 (1947)]. In another device, several receivers disposed inseries in the same beam path exhibiting selectivity to various gases areemployed. A similar device adapted only to the measuring of twocomponents has been marketed for some time [K.F. Luft, Z. Analyt. Chem.p. 164, 108 (1958)].

Although the above devices had some advantages, multicomponent devicesfor gas analysis have not been particularly successful in view of thelarge number of one-component measuring devices on the market.

In British Pat. No. 953,952, a gas mixture to be analyzed flows in twoequal streams into two measuring chambers of equal size throughcapillaries. By means of modulated irradiation'which is absorbed in themeasuring chambers, a pressure difference occurs which is detected by amembrane condenser. However, the capillaries are difficult tosynchronize and install in exactly the same manner to avoid unavoidablefluctuations of the flow ofthe gases, which produces interferingpressure differences at the condenser.

Today, various analytical methods are available, using gaschromotography and mass spectroscopy, which provide very wide limitswith respect to the number and type of components which can be detected.However, even with such available methods, there are many analyticalproblems wherein an efficient method of infrared multicomponent analysiswould be preferred. Thus, where three or more comdecomposition productsduring the heating of a coal sample.

SUMMARY OF THE INVENTION The invention provides a device formulticomponent gas analysis using a nondispersive infrared procedure,wherein the optical-pneumatic effect, produced by the absorption ofspecific bands of infrared radiations by individual components of a gasmixture, is measured. The device provides a means whereby only a verysmall portion of the gas mixture is required for the measurement, and isparticularly advantageous in analytical measurements of flowing gasessince all the gas flow need not be passed through the measuring chamberas is the case with other prior art devices.

The device of this invention is adapted to dampen pneumatic interferenceimpulses present in measuring systems for gases, especially flowinggases.

The device accomplishes the above by being pro vided with means forpermitting diffusion of a small portion of the gas mixture to bemeasured into a measuring chamber. The diffusion rate of the gas intothe measuring chamber is predetermined by the use of porous materials.By the use of the porous materials, the momentary optical-pneumaticeffect produced in the measuring chamber is not affected.

The device is capable of being selective to the various individual gascomponents of a gas mixture by means of optical filters. Theconcentration of the radiation within a small measuring chamber volume,on the order of a few tenths of a cubic centimeter, makes it possible toachieve a high degree of measuring sensitivity with a wide extension ofthe measuring range. The measurement is accomplished with a smallconsumption of the gas mixture to be analyzed and the response time issufficient even in view of the fact that the gas mixture replenishmentin the measuring chamber is by diffusion.

DETAILED DESCRIPTION OF THE INVENTION The invention will be describedand more fully understood by reference to the drawings which illustrateembodiments of the invention.

FIG. 1 shows schematically one embodiment of a gas analysis deviceaccording to the invention;

FIG. 2 shows a top view of the diaphragm wheel of the device of FIG. 1;

FIG. 3 shows a top view of an alternate diaphragm wheel to be used withthe device of FIG. I; and

FIG. 4 shows another embodiment of the gas analysis device according tothe invention.

Referring to the embodiment of FIG. 1, the numeral 10 generallyindicates a cuvette unit and the numeral 30 a receiver unit. A radiationsource 11 is optically v aligned with the cuvette unit 10 and adiaphragm wheel or shutter 13 is disposed between them driven by motor12 at a constant rotating speed.

The cuvette unit 10 comprises left and right semi-cuvettes 14 and 15,and a diffuser or filter cuvette l6 having a conical configuration.

The receiver unit 30 which can be a block of metal and the likecomprises a measuring chamber 17 disposed in optical alignment with thecuvette unit 10. The bottom of the chamber 17 comprises a disc 18 of aporous material such as a sintered metal connected directly to a duct 19through which the gas to be measured is carried. An identical comparisonchamber 20 is disposed symmetrically to the chamber 17 and alsocomprises a disc 18a of porous material connected directly to the duct19.

'Channels 21 and 22 connect chambers 17 and 20 respectively with ameasuring .twin diaphragm condenser 23 which serves to measure thepressure difference between the two chambers. Adjustable buffer volumes24 and. 25 are provided on both sides of the condenser which serve tobalance the pneumatic time constants on both sides of the diaphragm fromthe effects of pneumatic interference impulses which still remain inspite of the symmetry of the measuring system.

The gas to be measured or analyzed flows into duct 19 from capillary 26and then into a buffer volume chamber 27 from which it dischargesthrough capillary 28. Porous materials such as porous metal plates 31and 32 are employed for feeding and discharging the gas, which furtherdampen the effects of pneumatic interference impulses.

The signals obtained from the condenser 23 are fed to an amplifier andmeasuring instruments (not shown) from the diaphragm 33 and the twocounter electrodes 34 and 35.

In the optical alignment of the system, there are provided radiationpermeable windows 36,37,38,39,40, and 41.

The use of the device is particularly adaptable to degasificationprocesses wherein several gaseous decomposition products are formed andit is desired to measure the velocity of formation of one or more of theproducts. For example, in the degasification of coals, it may be desiredto measure the concentration of one or all of the components of thevarious gases being evolved, such as C0, C CH etc.

In order to sensitize the device to a specific gas, such as for example,CO cuvette 15 is filled with CO and cuvette 14 is filled with nitrogen.Filter cuvette 16 is filled with all the components in the gas mixturewhich absorb the specific radiation to be used, except the component tobe measured, in this case, CO With exact symmetry of the device, theradiation entering measuring chamber 17 is modulated only in the rangeof the absorption bands of the sensitizing gas, i.e., CO Since thepneumatic measuring system responds only to alternating pressures,pneumatic signals are produced when radiations are alternately passedthrough cuvettes 14 and 15 by the rotation of wheel 13 or 13a, and areabsorbed in chamber 17 in relation to the concentration of the specificgas for which the device is sensitized, i.e., CO

The radiations used are preferably infrared radiations and theradiation-absorbing gas for which the.

device is sensitized will absorb only a very small wavelength range ofthe total radiation emitted by the radiation source 11. When radiationspass through cuvette 15, the CO absorbs practically all the radiationwithin the absorption bands of CO whereas radiations passing throughcuvette 14, containing nonabsorbing nitrogen, are unweakened. Asradiations pass from cuvette 14 into the filter cuvette 16, which doesnot contain any CO radiations which may be absorbed by the gasses otherthan CO are filtered in order that they may not be absorbed in the gasto be measured flowing through duct 19 and entering chamber 17.Radiations passing through cuvette l4 and filter cuvette 16 will containno radiations which will be absorbed by the gas mixture in 17 becauseall the pertinent radiations will have been previously absorbed.

Accordingly, radiations passing into chamber 17 from cuvette 15 are onlyabsorbed by the CO content of the gas therein. Thus, the gas mixture in17 is heated in accordance with its CO content which results in apressure rise. Y a

As the radiations emitted by 11 are alternately passed through cuvettesl4. and 15 a pressure difference will result in chamber 17 which willvary in accordance with the CO content of the gas which has enteredchamber 17 from duct 19. Gas from duct 19 also enters chamber 20 whichdoes not receive any radiations. Accordingly, the gas pressure in bothchambers 17 and 20 are the same except when radiations enter chamber 17.The pressure differences which result in the two chambers are measuredby the diaphragm condenser 23, which responds to the alternatingpressures. The pneumatic signals are produced by the absorption ofradiation in chamber 17 corresponding to the CO content. The signals.are converted into periodic capacitance variations in the usual manner,amplified and recorded by usual means.

The decided advantage of the above described system is the fact that nopressure differences occur between chambers 17 and 20 due tofluctuations in the gas flow through duct 19. The same static pressureis always automatically attained in both chambers because the gasreplenishing the chambers for measuring purposes from duct 19 enters bydiffusion through porous discs 18 and 18a. Thus, the total gas flow doesnot pass directly through the measuring chambers as is the case withprior systems using capillaries and therefore, the difficultiesmentioned heretofore do not occur.

The rate of diffusion of gas through discs 18 and 18a dependssubstantially only on the sum of the areas of all the pores. The discscan be manufactured in thicknesses as small as 0.l mm. Porous plates canbe manufactured wherein the sum of the pore areas is 20 percent of thesurface of the plate. The decisive factor, however, is that the totalpore area has little dependence on the size of the individual pore area.Thus, if a porous plate is used having a surface area of l cm adiffusion is produced as if an opening of H5 cm were present. Throughsuch an opening, using measuring chambers having a small volume of about1 cc., the gas exchange betweenthe duct and chambers takes place bymeans of diffusion within a few seconds.

The diffusion of the gas mixture from duct 19 into the chambers takesplace with a time constant V= chamber volume D diffusion constants e=.thickness of the porous body n pore number F= individual pore area Theequalization of the pressure resulting through radiation absorptiontakes place with a time constant H constantfactor p.= viscosity of thegas As the product n-F shows, the total pore area is independent of thepore-diameter d where the structure of the porous material is constantin total pore area. The diffusion time constant remains practicallyconstant with decreasing pore-diameter, while the pneumatic timeconstant increases proportionally with HF. Tests have shown that with aporous material of pore diameter of about 1 am value, one attains thetime constants suitable for the device of the invention and itspurposes. Thus, for example, there occurs with a chamber volume of 0.5cm, a pore areaof 1 cm of a porous material being 3 mm. thick and bout 1am pore diameter, a diffusion time constant of 5 sec and a pneumatictime constant of 0.1 sec, which with a modulation frequency of 6 Hzoccasions a signal loss of only a few percent.

The porous discs also have the advantage that the increased pressureproduced in chamber 17 by absorption of the radiation and heating of thegas is not equalized through the porous discs during the short radiationpulses of about 1 second.

In accordance with Poiseuilles law, the flow resistance of an individualpore is inversely proportional to the square of the pore area. Thus, areduction of the pore diameter to one-tenth and keeping the pore areaconstant, increases the flow resistance by a factor of lOO. Therefore,if the pore diameter is selected to be sufficiently small, the diffusionrate of the gas through the porous walls can be selected to accommodatea desired gas diffusion from the duct 19 into the chambers, while at thesame time avoiding the effects of fluctuations of gas flow in the ductand loss of any of the pulsating pressure in chamber 17 produced by theabsorption of radiation.

Porous metallic plates or discs are commercially available which have apore diameter of 0.0002 mm. which are satisfactory to meet the aboverequired conditions.

Referring to FIG. 4, another embodiment of the device of the inventionis shown. The numeral 110 generally indicates a cuvette unit and thenumeral 130 a receiver unit. A radiation source 111 is optically alignedwith the cuvette unit 110 and a diaphragm wheel or shutter 113 isdisposed between them driven by motor 112 at a constant rotating speed.The cuvette unit 110 comprises a pair of cuvettes 114 and 115.

The receiver unit 130, which can be a block of metal and the like,comprises a chamber 1 17 disposed in optical alignment with cuvette 114.Chamber 117 comprises a disc 118 of porous material connected directlyto a duct 1 19 through which the gas to be measured is carried. Anidentical chamber 120 is disposed symmetrically to chamber 117 on theopposite side of duct 119 and in optical alignment with cuvette 115.Chamber 120 comprises a disc 118a of porous material connected directlyto duct 119. Channels 121 and 122 connect chambers 117 and 120respectively with a measuring twin diaphragm condenser unit which is ofthe same construction and operation as the condenser 23 shown anddescribed in connection with the embodiment of FIG. 1.

The gas to be measured or analyzed flows from capillary 126 throughporous plate 131 into duct 119 and then into buffer volume chamber 127from which it discharges through porous plate 132 and capillary 128.

In the optical alignment of the system, there are provided radiationpermeable windows 136,137,137a,1 38,138a,141, and 141a.

The operation and sensitization of the device of FIG. 4 is similar tothe device of the embodiment of FIG. 1. Cuvette is filled with thespecific infrared absorbing gas, e.g. CO and cuvette 115 with anonabsorbing gas, e.g. nitrogen. When radiations are passed throughcuvette 115, the CO absorbs practically all the radiations within theabsorption bands of CO whereas radiations passing through cuvette 114are unweakened.

Radiations passing from cuvette 114 into chamber 117 are absorbed by theCO content of the gas in chamber 117 which has diffused therein throughporous plate 118 from duct 119. Radiations passing from cuvette 115 intochamber will not be absorbed by the CO content of the gas in chamber 120because the CO in cuvette 115 has already absorbed the pertinentradiations.

As radiations emitted by 111 are alternately passed through cuvettes 114and 115, by rotating wheel 113, a periodic pressure increase will resultin chamber 117 giving a pulsating signal which will vary in accordancewith the CO content of the gas flowing in duct 119 and diffusing intochambers 117 and 120. This pulsating signal is obtained from thediaphragm condenser unit as described heretofore.

The unique construction of the device providing for diffusion of gasesinto the chambers with the attendant advantages set forth is based onthe fact that the diffusion time constant remains the same independentof the cross-sectional area of the individual pores, when thecross-sectional area of the total pores is constant. However, thepneumatic time constant increases inversely proportionally to thecross-sectional area of the individual pore. Thus, it is possible tohave rapid gas replenishment in the chambers by diffusion, but thecompensation of the pressure in the chamber, which increases byabsorption of radiation, is sufficiently slow in relation to theduration of the radiation.

FIGS. 3 and 4 illustrate diaphragm wheels that may be used to modulatethe infrared radiations passing through the adjacent cuvettes and themeasuring chambers.

The device is adapted to be sensitized to a plurality of individualgases contained in a gaseous composition. Thus, a plurality of cuvetteunits 10, each sensitized to an individual gas, can individually bebrought into position between the radiation source 11 and the receiverunit 30.

What is claimed is:

1. A device for the detection and measurement of a gas contained in agas mixture by means of the opticalpneumatic effect produced by theabsorption of radiation, comprising the following means in opticalalignment:

a. radiation and filter means,

b. a pair of adjacent cuvettes, one of which contains the gas to bedetected and the second of which contains a nonradiation absorbing gas,

0. receiver means comprising a measuring chamber and a comparisonchamber,

d. duct means conducting said gas mixture through said receiver meansbetween said measuring and comparison chambers,

V e. porous means disposed between said duct and said measuring andcomparison chambers for diffusing a portion of said gas mixture fromsaid conduit to said chambers, and

f. pressure differential measuring means.

2. The device of claim 1 wherein filter cuvette means is disposedbetween said receiver means and said adjacent cuvettes.

3. The device of claim 1 wherein said duct means comprises capillary andporous means at the entrance and discharge portions of said receivermeans.

4. The device of claim 1 wherein said pressure measuring means comprisesdiaphragm condenser means and adjustable balance control means disposedon each side of said condenser.

5. The device of claim 1 wherein said porous means comprises sintermetal and is dimensioned with respect to pore diameter and pore numberwhereby gas exchange takes place rapidly through diffusion anddifferential pressure building up during a radiation period remainsapproximately maintained.

6. The device of claim 1 wherein said duct means comprises buffer volumemeans.

7. The device of claim 1 wherein said radiation means comprisesdiaphragm wheel means.

8. The device of claim 1 wherein said comparison chamber is not inoptical alignment.

1. A device for the detection and measurement of a gas contained in agas mixture by means of the optical-pneumatic effect produced by theabsOrption of radiation, comprising the following means in opticalalignment: a. radiation and filter means, b. a pair of adjacentcuvettes, one of which contains the gas to be detected and the second ofwhich contains a nonradiation absorbing gas, c. receiver meanscomprising a measuring chamber and a comparison chamber, d. duct meansconducting said gas mixture through said receiver means between saidmeasuring and comparison chambers, e. porous means disposed between saidduct and said measuring and comparison chambers for diffusing a portionof said gas mixture from said conduit to said chambers, and f. pressuredifferential measuring means.
 2. The device of claim 1 wherein filtercuvette means is disposed between said receiver means and said adjacentcuvettes.
 3. The device of claim 1 wherein said duct means comprisescapillary and porous means at the entrance and discharge portions ofsaid receiver means.
 4. The device of claim 1 wherein said pressuremeasuring means comprises diaphragm condenser means and adjustablebalance control means disposed on each side of said condenser.
 5. Thedevice of claim 1 wherein said porous means comprises sinter metal andis dimensioned with respect to pore diameter and pore number whereby gasexchange takes place rapidly through diffusion and differential pressurebuilding up during a radiation period remains approximately maintained.6. The device of claim 1 wherein said duct means comprises buffer volumemeans.
 7. The device of claim 1 wherein said radiation means comprisesdiaphragm wheel means.
 8. The device of claim 1 wherein said comparisonchamber is not in optical alignment.